Cross-Section of the Zone of Maturation

Cortex - layer beneath epidermis; contains the vascular bundles with xylem (carrying water and mineral ions), phloem (carrying nutrient-rich sap) and cambium between them (gives rise to new xylem and phloem cells)

Movement of Water and Mineral Ions into the Root

Water diffuses from soil into root through root hairs.

Ions enter by active transport.

The entry of water into the xylem vessels of the root builds up root pressure that aids in upward movement of water.

The Stem

Cell Wall and Cytoplasmic Pathways

After water and mineral ions have been taken into the root, they travel through the plant either between the cell walls of plant cells (Cell wall pathway) or from cell to cell through the cytoplasm (Cytoplasmic pathway).

Stem Structure

Epidermis - outer layer of the stem; may be covered by a layer called the cuticle for protection; replaced by bark in a woody plant

Cortex - layer beneath the epidermis

Vascular Bundles - These are groups of outer phloem (carrying sugar-rich sap), inner xylem (carrying water and mineral ions), and cambium in between (gives rise to both phloem and xylem vessels.
Xylem is composed of long dead water-filled xylem vessels whose cell walls are strengthened by lignin and whose ends have holes for water flow, tracheids, supporting fibres and parenchyma cells.
Phloem is composed of living sieve tubes, parenchyma cells (especially a special kind called companion cells, and supportive fibres.

Pith the inner area composed of large parenchyma cells which serve as storage places

Transport of Water, Mineral Ions and Nutrients in the Stem

Diffusion and osmosis allow water and mineral nutrients to travel up the xylem from root to leaf. Also root pressure and capillary action aid in this.

Diffusion and osmosis also allow the two-way movement of nutrients such as sugar through the phloem.

Turgor Pressure is the pressure within plant cells due to amount of water. A plant wilts when there is a lack of water and reduction in turgidity.

Other Features of Stems

Herbaceous and Woody Plants - Herbaceous plants without bark are often annuals (life cycle of one year) or biennials (life cycle of two years). Woody plants with bark are often perennials, lasting many years.

Annual Rings - yearly deposits of xylem

'Ring-barking' - the process of cutting a circle through the bark and phloem, and allowing the tree to die slowly

Stoma (Plural: Stomata) - pores in the lower epidermis that are surrounded by bean-shaped guard cells that contain chloroplasts

Action of Stomata

In general, stomata open in the presence of light and close in the dark.

The bean-shaped guard cells have thicker walls on the side toward the stoma than on the other sides.

As glucose is produced and builds up in the guard cells during photosynthesis, water is drawn into the cells by osmosis. This increases the turgor pressure and the guard cells change shape, opening the stoma.

Transport of Water, Mineral Ions and Nutrients in the Leaf

Transpiration is the water loss that occurs through the open stomata. This loss helps to draw water up through the xylem from the root.
Factors that affect transpiration are temperature, light intensity and duration, wind speed and relative humidity.

Diffusion, osmosis and capillary action also play a role in transport through the phloem and xylem of veins. Much of the glucose produced in the leaf in the day is converted to starch in the leaf. It is then converted back into glucose for transport in phloem. If stored in root or stem, it is changed back into starch in most plants. The movement of glucose is called translocation.

Guttation only occurs in some plants and is the loss of water through special pores at the ends of leaf veins. Droplets are produced as a result of root pressure.

Aquatic plants such as water lilies have the stomata on the upper side of the leaf.

Did You Know That...? The tips of the stinging hairs of stinging nettles are actually made of glass that the plants manufacture from silicon in the soil.